The device is patterned from a commercial thin-film LN on insulator (LNOI) wafer (supplied by NANOLN) with 605 \(\mathrm {nm}\) -thick MgO doped \(Z\) -cut LN film bonded to \(2\,\mathrm {m}\)  thick silicon dioxide (\(\mathrm {SiO_{2}}\) ) on a \(400\,\mathrm {m}\) -thick silicon handle. The fabrication process of the LN waveguide closely follows our previous work [1]}, [2]}. To achieve high fidelity poling, a thin layer of \(\mathrm {HfO_{2}}\)  (\(\sim \)  10 nm) is deposited on top of the fabricated photonic device using the atomic layer deposition (ALD) technique, with which the thickness of \(\mathrm {HfO_{2}}\)  can be controlled with nano-meter precision and keeps the poling parameters invariant in different fabrication runs. Since \(\mathrm {HfO_{2}}\)  is a high-k material, it also aids in confining the electric field during poling which is necessary for ensuring a good duty cycle. After oxide deposition, Ni electrodes were deposited and patterned on top of the waveguide using commonly used PMMA and lift-off process. An optimized poling sequence was then applied to create the desired poling pattern[1]}, [4]}. Afterwards the Ni electrodes and the interface oxide were sequentially removed by wet etching. Figure REF (b) shows the false-color SEM image of the poled waveguide, where an excellent poling fidelity with reversed domains as small as 0.8 \(\mathrm {m}\)  and a duty cycle close to \(40\%\)  are indicated.
